Electrostatics and counterions play important roles in many supramolecular processes, including surfactant adsorption and aggregation at interfaces. Here, we assess their influence on how the common surfactant cetyltrimethylammonium (CTA) interacts with fused silica/aqueous interfaces by determining thermodynamic, kinetic, and electrostatic parameters for CTA adsorption across a range of NaCl concentrations (10-500 mM NaCl) using second harmonic generation (SHG). Using vibrational sum frequency generation (SFG), we demonstrate that vibrationally resonant contributions and nonresonant background contributions to the SFG signal intensity that depend on the interfacial potential can be quantified simultaneously during the adsorption process, which provides insight into the nonequilibrium dynamics of CTA adsorption. By analyzing the adsorption free energies as a function of interfacial potential at these four salt concentrations, the charge density per adsorbate is determined, indicating that CTA coadsorbs with counterions at a ratio of approximately 4 to 3 (i.e., 4 CTA(+) ions for every 3 Cl(-) ion). The chemical (i.e., non-Coulombic) portion of the free energy is found to dominate the overall free energy of adsorption, indicating that CTA adsorption at these ionic strengths is primarily driven by the favorable hydrophobic interactions between interdigitated surfactant hydrocarbon chains in the adsorbed aggregate. By applying Gouy-Chapman-Stern theory to our data, an average charge density of 2.8(3) x 10(13) charges/cm(2), which corresponds to 0.7 to 1.7 molecules/nm(2), was obtained for the four NaCl concentrations.

The adsorption of Zn2+ to glucosamide-functionalized fused silica/water interfaces is studied using second harmonic generation (SHG). We characterize each step of the surface functionalization using vibrational sum frequency generation (SFG) and X-ray photoelectron spectroscopy (XPS), where specific vibrational modes in the C−H region and binding energies in the C1s and N1s region are determined to be indicative of glucose covalently tethered to the surface. We employ the SHG χ(3) technique to track Zn2+ adsorption and desorption directly at the glucosamide-functionalized fused silica/aqueous interface at pH 7 and 10 mM NaCl and determine the electrostatic and thermodynamic binding parameters using standard electrical double layer models to quantify the change in interfacial potential upon zinc adsorption. The results presented here allow for the possibility of 2:1 to 3:1 carbohydrate:metal coordination complexes and suggest the possibility for multivalent interactions which have not been observed with glucose in the bulk aqueous phase, where 1:1 complexes dominate. These findings suggest that interactions between metal ions and carbohydrate arrays may be much stronger at interfaces as opposed to the bulk phase, with direct implications for controlling and predicting coordination chemistry.

While many biogenic and anthropogenic organic constituents in the atmosphere are surface-active and chiral, the role of stereochemistry in heterogeneous oxidation chemistry in the atmosphere has not yet been evaluated. Here, we present nonlinear vibrational surface spectra of fused silica substrates functionalized with quinuclidine diastereomers during exposure to 10(11) to 10(13) molecules of ozone per cm(3) in 1 atm helium to model ozone-limited and ozone-rich tropospheric conditions. Kinetic studies show that diastereomers that orient their reactive C=C double bonds toward the gas phase exhibit heterogeneous ozonolysis rate constants that are 2 times faster than diastereomers that orient their C=C double bonds away from the gas phase. Insofar as our laboratory model studies are representative of real world environments, our studies suggest that the propensity of aerosol particles coated with chiral semivolatile organic compounds to react with ozone may depend on stereochemistry. We expect that the differences in chemical accessibility will lead to the enrichment of one oxidation product stereoisomer over the other. The oxidation products could be gaseous or surface-bound, indicating that kinetic resolution could lead to the stereochemical enrichment of the gas phase or the aerosol, which may have also been important in prebiotic chemistry. Implications of these results for chiral markers that would allow for source appointments of anthropogenic versus biogenic carbon emissions are discussed.

Important mechanistic differences regarding C=C double-bond oxidation processes under ozone-limited and ozone-rich reaction conditions for cyclohexene-functionalized fused silica substrates serving as model systems for studying heterogeneous C=C double bond oxidation chemistry in the troposphere are evaluated. By using broadband vibrational sum frequency generation (SFG), we track heterogeneous ozone reactions in real time. Ozone levels span three orders of magnitude and represent environments ranging from pristine remote continental regions to highly polluted urban centers, ranging from 30 ppb to 3 ppm (from 7 x 10(11) molecules cm(-3) to 7 x 10(13) molecules cm(-3)). We determine reaction rates and reactive uptake coefficients (gamma values). At these tropospherically relevant ozone levels, the heterogeneous reaction rates follow a Langmuir-Hinshelwood-type mechanism. The product formation rates, which we determine as a function of ozone concentrations, are found to be half of the olefin reaction rates. This ratio is consistent with the previously proposed reaction pathway involving the breaking of one C=C double bond containing two olefinic CH moieties to form a product containing only one methyl group and one polar carbonyl moiety. Contact angle histograms show that out of a total of 60 measurements, there are about 25 more measurements with contact angles up to ten degrees below the mean recorded prior to reaction when ozone levels resemble remote continental conditions (50 ppb) than when ozone levels resemble urban conditions (1 ppm). The implication of these results are that the methyl formation pathway in heterogeneous ozonolysis may be less favorable than the carboxylic acid- and secondary ozonide-production pathway for ozone-limited conditions (i.e., in the remote continental troposphere or during urban nighttime) as opposed to ozone-rich (i.e., polluted urban atmosphere) conditions.

The interaction of Zn2+ ions with undecanol-functionalized fused silica/water interfaces was studied directly at the aqueous/solid interface. We characterized the surface functionalization using vibrational sum frequency generation (SFG) and X-ray photoelectron spectroscopy (XPS). We then employed the SHG χ(3) technique to determine the degree of silane functionalization, track Zn2+ adsorption directly at the hydroxyl-terminated undecanol silane-functionalized fused silica/aqueous interface at pH 7 and 10 mM NaCl concentration, determine the electrostatic and thermodynamic binding parameters, quantify the change in interfacial potential upon zinc ion adsorption, and compare these values to our previous work with glucosamine-functionalized and bare fused silica/water interfaces. The results from the calculated adsorption free energy suggest that 2:1 hydroxyl/metal coordination complexes, which have not been observed with natural carbohydrates in the bulk aqueous phase, are possible in interfacial environments, with direct implications for controlling and predicting coordination chemistry.